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Residential Infill Options - Development Options while Retaining an Existing House

Are you looking to add more dwelling units to your property while keeping an existing house? Several options are available for your project.
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Additional floor area allowed through bonus

As an incentive to retaining an existing house, bonus floor area is available when additional units (up to a maximum of four) are added to a site with an existing residential structure. The existing residential structure can be converted to add units or accessory dwelling units (ADUs) can be added to the site. To qualify for this bonus, the existing residential structure be at least five years old and no more than 25 percent of the existing street-facing façade may be altered to add additional floor area.

Floor Area Ratio allowed when Units are added to an Existing House:

1Not applicable0.4 to 10.4 to 10.4 to 10.5 to 10.7 to 1
2Not applicable0.6 to 10.6 to 10.6 to 10.7 to 10.9 to 1
3Not applicable0.7 to 10.7 to 10.7 to 10.8 to 11 to 1
4 or moreNot applicable0.8 to 10.8 to 10.8 to 10.9 to 11.1 to 1

Adding Accessory Dwelling Units (ADUs)

A single ADU is allowed on any lot with a house, attached house or manufactured home. Two ADUs are also allowed on these lots, as long as the lot has frontage on a maintained street or self-contained pedestrian connection that connects to a maintained street. One ADU may be part of the house and one may be detached. There may also be a house and two detached ADUs. Two ADUs inside or attached to a house are not allowed. It is possible for the two detached ADUs to be attached to each other – either side-by-side or stacked.

Diagram showing both an internal and external or detached ADU.
  • Minimum lot sizes required for (2) ADUs are:
    • R20: 12,000 sq. ft.
    • R10: 6,000 sq. ft.
    • R7: 4,200 sq. ft.
    • R5: 3,000 sq. ft.
    • R2.5: 1,500 sq. ft.
  • Additional floor area ratio is earned for three or more units.
  • If the ADU is detached or connected to the house with a breezeway, additional development standards in 33.110.245 apply. They must also be located 40 feet from a front lot line or located behind therear building wall of thehouse (see 33.205.040.C.3).
  • ADUs are limited to 800 square feet of living area (or 75 percent of the living area of the primary structure, whichever is less), unless they are added to the basement of an existing house that is at least five years old.
  • If two ADUs are proposed, one of the units on the site must meet the visitability standards in 33.205.040.C.5.
  • System Development Charges (infrastructure impact fees charged with building permits) may be waived for one or two ADUs where none of the units on the lot are used as an Accessory Short Term Rental.

Detached Duplex

A second, detached primary dwelling unit may be added to a lot when there is already a house on the lot that is at least five years old. The second dwelling is limited in height to 25 feet.

  • As a second primary dwelling unit, System Development Charges apply to the new development.
  • With development of a Detached Duplex, the two structures may be proposed for individual lots through a Middle Housing Land Division.

Lot Confirmations

A lot confirmation is an administrative review that verifies one or more lots or lots of record have legal status as a property that is eligible for development under the zoning code. It is faster and less expensive than a land division, but is only possible when the property is made up of previously platted lots (e.g. legal description is Portland Addition, Block 24, Lots 3-5) or lots of record created via deed before July 26, 1979. An existing house may be located on a property that contains one or more lots that can be split off to be sold or proposed for new development.

  • A survey is required to show the distances between buildings on the lot and the property line that is being confirmed.
  • Existing development must remain in conformance with the development standards of the zoning code after the Lot Confirmation unless an Adjustment land use review or Property Line Adjustment is approved. Note that floor area ratio (FAR) is not a standard that can be adjusted or modified through a land use review.
  • A lot confirmation does not verify accessibility to services such as water or sewer; it only confirms that the property meets zoning code standards to be eligible for development. Providing services to confirmed lots can add considerable expense to new development.
  • In the single-dwelling zones, each lot must have frontage on a street, and each lot must meet the standards of 33.110.202, When Primary Structures are Allowed.

Small flag lots

In the R5 and R2.5 zone, small flag lots may be created via Property Line Adjustment (PLA) by taking advantage of an underlying platted lot that would otherwise be undevelopable due to the location of an existing residence. This option allows for development of a smaller home on a lot that may be sold separately while retaining the existing residence.

  • Existing houses are allowed to exceed the maximum floor area ratio allowed by the zone when the flag lot reduces the size of the lot with the house. Future alterations may not move the development further out of conformance and new development must comply with the maximum floor area ratio for that lot.
    Diagram and overhead picture of a lot showing an example of a flag lot.
  • The minimum lot area for each reconfigured lot is 1,600 square feet (excluding pole portion) and the new flag lot must be less than 3,000 square feet (including the pole portion).
  • The flag pole access to the lot is required to be 12-feet wide if providing vehicle access or 10-feet wide if not.
  • Residential structures located on small flag lots, which are less than 3,000 square feet in area, are subject to additional size, height limit, and design standards (33.110.255.C.2).